In 1927, the term „picture element“, later abbreviated to „pixel“, appeared for the first time in the American technology magazine Wireless World. Today, pixels are everywhere: in computer screens and television sets, where they create colourful images; but also in cameras, where they capture images. In any case, however, they do one or the other—either they control light, as in the case of a display, or they analyse it in a camera sensor. Until now, there have been no pixels that could do both.
A research team led by David Norris, Professor at the Optical Materials Engineering Laboratory at ETH Zurich, has now developed such pixels for the first time. These pixels can both steer light and analyse it. Not only the intensity of the light, but also its oscillation phase and polarisation can be controlled and analysed. In the future, such so-called bidirectional pixels could lead, for instance, to the development of camera–displays that combine the two functions in a single device.
The new results, which have recently been published in the scientific journal external page Nature, are based on a fundamental physical effect: the so-called interference of light waves. When light is scattered by a surface, the waves originating from different points on the surface overlap. The shape of the surface determines the oscillation phases with which the waves propagate further. If the phases are equal, the light waves reinforce each other, but if they are opposed, the waves cancel out.
Norris and his collaborators use this effect to precisely control light with wave-shaped sculpted surfaces. They developed this processing method, which is precise to within a few nanometres, already a few years ago. For steering, the pixel—that is, the area on the chip where the material has been processed—first transforms the incoming light into a surface wave (a so-called surface plasmon polariton) propagating along the surface of the chip.
At a different position within the pixel, the surface wave is scattered back out of the material as a light wave. Through interference of the light waves, patterns and images can be created. Using mathematical Fourier analysis, the researchers can calculate what these images will look like and what kind of surface pattern is needed for a specific image.